The present invention relates to novel pyridazinone derivatives having cardiotonic activity and to pharmaceutically acceptable acid addition salts thereof.
As the compounds having pyridazinone ring and which exhibit cardiotonic activity, there have been disclosed pimobendan of the following formula: ##STR2## [Japanese Published Unexamined Patent Application No. 33479/80 (U.S. Pat. No. 4,361,563)], LY195115 of the following formula: ##STR3## [Japanese Published Unexamined Patent Application No. 22080/86 (EP 161918A)], CI-930 of the following formula: ##STR4## [Japanese Published Unexamined Patent Application No. 74679/83 (EP 75436A)], etc. Further, as the compounds having pyridone ring and which exhibit cardiotonic activity, milrinone of the following formula: ##STR5## [Japanese Published Unexamined Patent Application No. 70868/82 (U.S. Pat. No. 4,297,360)], etc. have been disclosed.
Cardiotonics have been used for the treatment of chronic cardiac insufficiency. However, the known cardiotonics are still unsatisfactory in effectiveness and duration of effectiveness.